Most reciprocating aircraft engines have two spark plugs per cylinder. What are the reasons for this?
Bold Method has a great post today about why aircraft engines have more than one spark plug per cylinder. There are a few basic reasons.
Having two sparks plugs is more reliable. If one spark plug becomes inoperative for some reason, there's a second to provide the spark for the power stroke.
It should also be noted that the spark plugs are usually powered by dual, independent magnetos or, in some cases, an electronic capacitor discharge ignition. The redundancy of the ignition system adds to the reliability.
Having two spark plugs means the flame ignites from two points leading to more power per power stroke. This point is demonstrated during run-up. When the magneto check is done the engine RPMs drop when running on a single mag and set of spark plugs.
Two sparks plugs and a two flame front leads to a more even burn of the fuel air mixture for a smoother running engine.
Aircraft engines burn leaded fuel which can lead to getting lead deposits on the spark plugs. Having dual plugs leads to a more complete burn which can prevent the deposits from forming.
Reliability is the key thing. The ignition system of a gasoline engine (the "magnetos" in most aircraft) is absolutely critical to the engine's operation, and thus the safety of the flight of the aircraft possessing it. A small break in a wire, or fouling of one plug, could cause cylinder misfires or even a total engine failure. To guard against this, dual independent ignition systems are incorporated into aviation engines to allow for continued operation in the event that any component of one ignition system fails.
Efficiency is a secondary benefit. Two spark plugs per cylinder means the spark initiates combustion at two points within the volume of the cylinder. Combustion rate of a vaporized fuel is typically a function of "surface area" of the region of combustion, so two sparks equals double the combustion surface area. This produces faster and more complete combustion, increasing power at a given fuel flow rate. One of the rudimentary "runup" tests of a piston engine involves selecting one or the other magneto, and watching the performance of the engine. The normal result with a healthy ignition system is a slight reduction in power, as indicated (in a fixed-pitch prop at least) by lower RPM. A severe power reduction, rough operation or an engine quit is a sign something's wrong on that side of the ignition system.